Air conditioning must be regulated in a manner that makes it possible to servo-control the temperature inside the premises to a reference temperature. The regulation must also ensure that initial stages during which air conditioning is being established take place quickly. Thus, the regulation of air conditioning in rail cars must enable air conditioning to be brought into operation quickly and then ensure a constant temperature for passenger comfort.
Air conditioning is often regulated by means of proportional-integral-differential (PID) action systems which do not give full satisfaction, particularly in railway applications. When there is a large difference between the reference temperature and the outside temperature, PID type regulation gives rise to too long a delay in reaching steady conditions and expends too much energy. Generally, when the system is switched on, either the reference value is overshot by a wide margin, thereby giving rise to a pointless loss of energy and to a temperature stabilization time that is too long, or else there is no overshoot, but the waiting time before reaching the reference temperature is much too long.
In practice, the coefficients of the PID system are adjusted manually prior to installing the system in a vehicle, in the hope of achieving the best compromise between the phenomenon of integral action becoming saturated and the difficulty of adjusting differential action.
Regulation of air conditioning based on a PID type system does not perform the function of bringing the air conditioning into operation quickly without overshoot in spite of numerous design tricks being used (desaturation of integral action, change of regulator coefficients in operation) which merely lead to a structure that is complicated and difficult to develop. This problem is particularly difficult for premises where mean operation of the air conditioning assembly is related to the non-linearity of the system (large differences in the dynamic behavior between stages during which temperature is rising and stages during which temperature is falling). Such non-linearity cannot be taken into account by a PID regulator.